A soldier stands near broken windows after explosions hit the Zaventem airport near Brussels on March 22. (Photo: Francois Lenoir/Reuters)

Brussels, where the Paris attacks were believed to have been planned, is a particular hotbed of terrorist activity. About 470 jihadis from Belgium are believed to have gone to fight in the Syria-Iraq conflicts in recent years, with some 118 reported to have returned to Belgium, according to a report last December by the Soufan Group, an international consulting firm.

U.S. officials, however, acknowledge that they had no specific warning about the Tuesday attack, raising continuing questions about how members of the Belgium terrorist network have been able to communicate without detection by Western intelligence services.

National Security Agency Director Adm. Mike Rogers recently told Yahoo News in an interview that the agency believes the Paris attackers used encrypted apps — a conclusion that has been confirmed by French authorities, according a report Sunday in the New York Times.

A U.S. source told Yahoo News on Tuesday that intelligence officials believe the Paris-Brussels networks have communicated through a “combination” of encrypted communications and highly disciplined face-to-face communications.

As further evidence that U.S. officials had no inkling about an imminent attack in Belgium, Nicholas Rasmussen, director of the National Counterterrorism Center, had just been in Brussels on Sunday, speaking at the German Marshall Fund security forum.

“They didn’t have a clue about this,” one U.S. counterterrorism source told Yahoo News.